Method and device for making substrates

ABSTRACT

The invention concerns a method for making substrates, in particular for optics, electronics or optoelectronics, characterised in that it comprises an operation which consists in implanting ( 100 ) atomic species beneath the surface of a material in the form of a cylindrical ingot ( 1 ), at a depth of implantation distributed about a certain value by bombardment of said atomic species on a zone of the ingot ( 1 ) cylindrical surface, and an operation which consists in removing ( 300 ), at a separation depth located proximate to the depth of implantation, the layer ( 2 ) of material located between the surface and the separation depth, to remove said layer ( 2 ) from the rest of the ingot ( 1 )

[0001] The invention relates to the fabrication of substrates, inparticular for optics, electronics or opto-electronics.

[0002] Substrates for use in the aforementioned fields are generallyobtained industrially by cutting up ingots. In the case ofmonocrystalline silicon, for example, the ingots are obtained from abath of molten silicon by the Czochralski drawing method (referred tohereinafter as CZ drawing) or from a polycrystalline ingot by the zonefusion method (referred to hereinafter as FZ drawing). These growingmethods produce cylindrical ingots which are then cut into slicesperpendicular to the axis of the cylinder, generally using an internalcut circular saw.

[0003] However, the above methods do not produce substrates withsatisfactory dimensions for some applications. This applies inparticular in the field of fabrication of large substrates that can beused to make flat or other shape display screens or solar cells.

[0004] To obtain larger monocrystalline silicon substrates, the documentFR 2 752 768 proposes cutting ingots parallel to their longitudinalaxis.

[0005] One object of the invention is to propose another way offabricating substrates from ingots.

[0006] Another object of the invention is to obtain substrates of amonocrystalline material, such as monocrystalline silicon, for example,with lower production costs than with prior art methods.

[0007] That object is achieved by a method of fabricating substrates, inparticular for optics, electronics or opto-electronics, the method beingcharacterized in that it includes:

[0008] an operation of implanting atomic species under the surface of amaterial in the form of a cylindrical ingot at an implantation depthdistributed around a particular value by bombarding said atomic speciesonto an area of the cylindrical surface of the ingot, and

[0009] a detachment operation at a detachment depth in the vicinity ofthe implantation depth of the layer of material between the surface andthe detachment depth to detach that layer from the remainder of theingot.

[0010] Thus in accordance with the invention a substrate is obtained bypeeling off the surface layer of a cylindrical ingot parallel to theaxis of the cylinder.

[0011] It must be understood that the term “substrate” is usedthroughout this text in the widest sense of the term, in other words todesignate either an element of material able to serve as a support foranother element or a thick or thin, rigid or flexible, etc. film orlayer.

[0012] The terms “cylinder” and “cylindrical” must also be understood intheir primary sense. In this sense, a cylinder is a solid body generatedby a straight line that moves parallel to itself along the surface of acurve. In this text a cylinder can therefore have a round cross sectionor a polygonal cross section.

[0013] The method according to the invention advantageously enablescontinuous fabrication of substrates.

[0014] Because the method according to the invention can be implementedcontinuously, it increases the productivity of the fabrication ofsubstrates and therefore reduces production costs. The method isparticularly beneficial if it is required to fabricate monocrystallinesilicon substrates at low cost, for example.

[0015] The method according to the invention has the followingadvantageous features, independently or in combination:

[0016] the implantation of the atomic species is effected bycontinuously bombarding the cylindrical surface of the ingot with alocalised beam that is swept in the longitudinal direction of the ingotwhile the ingot rotates about an axis parallel to the cylindricalsurface;

[0017] the implantation of the atomic species is effected bycontinuously bombarding the cylindrical surface of the ingot with a beamof elongate cross section corresponding to a given area while the ingotrotates about an axis parallel to the cylindrical surface;

[0018] the implantation of the atomic species is effected by bombardmentcorresponding to a given area of successive zones adjacent thecylindrical surface of the ingot while the ingot rotates about an axisparallel to the cylindrical surface of the ingot between eachbombardment and the next;

[0019] the implantation of the atomic species is effected bycontinuously bombarding the whole of the cylindrical surface of theingot, for example by immersing the ingot in a plasma;

[0020] it further includes an operation of heating the cylindricalsurface; the heating operation can be carried out before, during orafter implantation; the heating operation can be carried out by theimplantation itself; the heating operation reduces the necessary dose ofatomic species implanted and/or encourages in situ healing of someimplantation defects;

[0021] it includes an operation of transferring the layer of materialbetween the cylindrical surface of the ingot and the detachment depthonto a support;

[0022] it includes an operation of pressing the layer of materialbetween the cylindrical surface of the ingot and the detachment depth bymeans of a roller; this pressing operation causes a thermal shock if theroller is cooled or heats the ingot if it is heated and/or appliesmechanical pressure and/or shear stresses to encourage and/or causedetachment from the ingot of the layer of material between thecylindrical surface of the ingot and the detachment depth;

[0023] the support is adhesive;

[0024] it includes an operation of covering the layer of materialbetween the cylindrical surface of the ingot and the detachment depthwith a liquid phase or gas phase deposit;

[0025] the material is silicon;

[0026] the atomic species comprise hydrogen ions;

[0027] the atomic species comprise doping ions such as phosphorus,arsenic or boron ions;

[0028] it includes operations of applying a layer to each face of thelayer of material between the cylindrical surface of the ingot and thedetachment depth by rolling those layers between rollers;

[0029] it includes an operation of transferring at least one layercomprising circuitry patterns onto the layer of material between thecylindrical surface of the ingot and the detachment depth.

[0030] For the fabrication of display screens and solar cells inparticular, amorphous or polycrystalline silicon deposited on a glasssubstrate is often used, because monocrystalline silicon can bedeposited on glass, on quartz, etc. at present only by layer transfertechniques using a monocrystalline silicon substrate to form the layerthat is transferred onto the glass substrate. The diameter ofmonocrystalline silicon substrates is limited at present to 200 mm oreven 300 mm.

[0031] Using the method according to the invention, it is possible tofabricate larger monocrystalline silicon substrates. Even if themonocrystalline silicon substrates obtained with some variants of themethod according to the invention do not have a perfect crystallineorientation, they have better qualities than amorphous orpolycrystalline silicon. Thus when they are used to make flat screensthey offer an improvement in terms of integration density (number ofpixels per unit surface area), screen refresh rate, etc. When they areused to make solar cells they increase the efficiency of photo-electricconversion.

[0032] Whether the substrates are small or large, the fact that themethod of fabricating them can be implemented continuously reduces theircost.

[0033] Note that the initial diameter of the ingot is of littleimportance, but its length is more important. As a general rule, thegreater the diameter of an ingot the shorter its length. Accordingly,depending on the intended applications, it is preferable to use an ingotof smaller diameter, because it is easier to obtain in a greater length.However, the layer made by the method according to the invention isobtained from the cylindrical surface of an ingot. In some embodimentsof the method according to the invention, this layer can initially begiven a curvature that can prove critical in some circumstances. Forexample, if the layer is to be stored on a cylinder with the curvaturereversed, mechanical stresses can be generated that degrade the qualityof the layer, or even cause it to break. As the diameter of the ingotincreases, these curvature problems can become less severe.

[0034] It can also be beneficial to use large-diameter ingots if it isnecessary to reduce the period in the resulting layer of variation ofits crystalline orientation with the direction in which it is wound.

[0035] What is more, the curvature and crystallographic orientationproblems referred to above are considerably reduced, or evennon-existent, if the method according to the invention is used withingots which have a square cross section, for example. In this case, thelayer made by the method according to the invention is obtained fromplane faces whose crystallographic orientation is clearly defined.

[0036] A 200 mm diameter ingot is typically 1.5 m long. As a generalrule, before they are used in the method according to the invention,these ingots are cut into 40 to 50 cm lengths.

[0037] What is more, an ingot as drawn has an ill-defined exterior shape(undulating diameter, etc). During a preliminary step of the methodaccording to the invention, the ingot is turned or machined to obtain aningot in the form of a regular cylinder or having a polygonal crosssection. The turning or machining operation is carried out before orafter the cutting operations previously referred to.

[0038] With silicon ingots, a layer of the order of 10 μm thick isobtained by implanting hydrogen ions with energies of the order of 1MeV. However, what is essential is to have a sufficiently rigid mass ofmaterial between the detachment depth and the surface of the ingot toavoid problems associated with the fragility and deformability of thelayers.

[0039] The detachment depth is advantageously determined so that thecontinuous layers formed by the method according to the invention areself-supporting.

[0040] In an advantageous variant of the method according to theinvention the layer is reinforced to prevent problems associated withits fragility or deformability by depositing a film before thedetachment operation or even before the heating operation, if any, oreven before the implantation operation. This variant is particularlyadvantageous when it is required to fabricate layers that are too thinto be self-supporting. In the case of silicon, for example, a 10 μmdeposit of SiO₂ proves sufficient to reinforce the mechanical strengthof the layer formed (a material other than SiO₂ can equally well beused). As a general rule, and therefore not only for applications of themethod according to the invention to the fabrication of silicon layers,deposition methods of the epitaxial, atomisation, paint, spray, etc.type are also feasible.

[0041] The amounts of the atomic species implanted are advantageously ofthe order of 10¹⁷ to 10 ¹⁸/cm². With these amounts, and a depth ofpenetration of the order of one to several tens of microns, it ispossible to separate the layer of material between the surface and thedetachment depth from the remainder of the ingot with no additionalheating operation and with the application of limited stresses or evenno stresses.

[0042] As a general rule, if stresses are applied to the layer, they areadvantageously mechanical stresses (shear, tension, compression,ultrasound, etc.), electrical stresses (electrostatic or electromagneticfield), thermal stresses (radiation, convection, conduction, etc.), etc.Applying stresses can also entail directing onto the layer/ingotdetachment interface a jet of fluid (liquid or gas) that is eithercontinuous or varies in time. Thermal stresses in particular can bederived from the application of an electromagnetic field, an electronbeam, thermo-electric heating, a cryogenic fluid, a supercooled liquid,etc.

[0043] Another aspect of the invention provides a device for fabricatingsubstrates, in particular for use in optics, electronics oropto-electronics, which device is characterized in that it includes:

[0044] means for implantation of atomic species under the surface of amaterial in the form of a cylindrical ingot at an implantation depthdistributed around a particular value,

[0045] detaching means for detaching a layer of material at a detachmentdepth in the vicinity of the implantation depth, and

[0046] rotation means for rotating a cylindrical ingot of the materialabout an axis parallel to the cylindrical surface of the ingot.

[0047] The above device implements the method according to the inventionas previously described. It advantageously includes means for holdingthe layer of material between the cylindrical surface of the ingot andthe detachment depth to gather up said layer after it is detached fromthe ingot. The holding means advantageously include a plurality ofreversible gripping means distributed over roller drive means. Theprinciple of such gripping means is known in the art. Such grippingmeans employ pressure differences, electrostatic forces, etc., forexample.

[0048] Other aspects, objects and advantages of the invention willbecome apparent on reading the following detailed description. Theinvention is explained with reference to the drawings, in which:

[0049]FIG. 1 is a diagrammatic perspective view of an ingot subjected toion implantation and to the detachment of a layer of material by a firstembodiment of the substrate fabrication method according to theinvention;

[0050]FIG. 2 is a diagrammatic view of one example of a substratefabrication device according to the present invention, in cross sectionrelative to the axis of the cylinder of the ingot shown in FIG. 1;

[0051]FIG. 3 is a diagrammatic view of the use of a second embodiment ofthe method according to the present invention, in cross section relativeto the axis of the cylinder of an ingot like that shown in FIGS. 1 and2;

[0052]FIG. 4 is a diagrammatic view of the use of a third embodiment ofthe method according to the present invention, in cross section relativeto the axis of the cylinder of an ingot like that shown in FIGS. 1 to 3;

[0053]FIG. 5 is a diagrammatic view of the use of a variant of theembodiment of the method according to the present invention shown inFIG. 4, in cross section relative to the axis of the cylinder of aningot such as that shown in FIGS. 1 to 4;

[0054]FIG. 6 is a diagrammatic view of the use of another variant of theembodiment of the method according to the present invention shown inFIG. 4, in cross section relative to the axis of the cylinder of aningot such as that shown in FIGS. 1 to 5;

[0055]FIG. 7 is a diagrammatic view of the use of a fourth embodiment ofthe method according to the present invention, in cross section relativeto the axis of an ingot like that shown in FIGS. 1 to 6;

[0056]FIG. 8 is a diagrammatic view of the use of a fifth embodiment ofthe method according to the present invention, in cross section relativeto the axis of an ingot like that shown in FIGS. 1 to 7;

[0057]FIG. 9 is a diagrammatic view of the use of a variant of thefourth embodiment of the method according to the present invention, incross section relative to the axis of an ingot like that shown in FIGS.1 to 7;

[0058]FIG. 10 is a diagrammatic view of the use of a sixth embodiment ofthe method according to the present invention, in cross section relativeto the axis of an ingot like that shown in FIGS. 1 to 9;

[0059]FIG. 11a is a diagrammatic view of a substrate obtained with aseventh embodiment of the method according to the invention, in sectionperpendicular to the surface subjected to bombardment by the methodaccording to the invention, and FIG. 11b is a diagram showing theconcentration profile of the atomic species implanted as a function ofthe depth of implantation in the substrate shown in FIG. 11a;

[0060]FIG. 12a is a diagrammatic perspective view of three layersintended to be superposed in an eighth embodiment of the methodaccording to the invention and FIG. 12b shows the structure obtainedafter assembling the three layers shown in FIG. 12a;

[0061]FIG. 13 is a diagrammatic view of the use of a ninth embodiment ofthe method according to the present invention, in cross section relativeto the axis of the cylinder of the ingot shown in FIGS. 1 to 10; and

[0062]FIG. 14 is a diagrammatic view of a square cross section ingotused in a variant of the present invention, in cross section relative tothe axis of the ingot.

[0063] The method according to the present invention is describedhereinafter in the particular context of obtaining monocrystallinesilicon substrates from an ingot obtained by CZ or FZ drawing. Siliconhas been chosen because it is by far and away the most widely usedmaterial in the field of micro-electronics. However, the invention isnot limited to this material. The invention applies generally to ingotsof any monocrystalline, polycrystalline or amorphous materials, inparticular semiconductors.

[0064]FIG. 1 shows an ingot 1 of monocrystalline silicon obtained by theCZ or FZ drawing process. It is approximately the shape of a circularcylinder with an axis X-X. The ingot 1 initially has a diameter of 200mm and a length L=1.5 m and is usually cut into lengths. The drawingprocess is chosen to obtain an ingot 1 in which the faces perpendicularto the axis of the cylinder are oriented parallel to the <1, 0, 0>crystallographic plane. The <0, 0, 1> plane 3 and the <0, 1, 0> plane 5are therefore parallel to the axis X-X of the ingot 1.

[0065] Ten examples of implementation of the method according to theinvention are described hereinafter.

EXAMPLE 1

[0066] In the first example, shown in FIG. 1, the method according tothe invention includes an operation 100 of implanting atomic species anda detachment operation 300.

[0067] In this example, the atomic species are H⁻ ions. They areimplanted with a high energy. The beam made up of these ions is elongatein the longitudinal direction of the ingot. To obtain a 10 μm thicklayer 2 of silicon the H⁻ ions are accelerated with an energy of 725keV. The amount of H⁻ ions implanted is 1.21×10¹⁷/cm².

[0068] The implantation operation 100 is carried out by sweeping a beamof accelerated atomic species over the surface of the ingot 1, over thewhole of its length, matching the rotation speed of the ingot 1 to thewidth of the beam and to the sweeping rate to obtain the appropriatedose.

[0069] The implantation depth Rp varies according to thecrystallographic orientation of the implanted surface relative to theincident beam of atomic species. In applications in which variations inthe thickness of the layer 2 are critical, modulation of the thicknessof the layer 2 obtained is advantageously avoided by modulating theimplantation energy as a function of the rotation angle. Note, however,that if an ingot 1 which has a square cross section is used, theseproblems of thickness variations can be reduced or even eliminated,because the implantation can be effected on faces whose crystallographicorientation is clearly defined.

[0070] The implantation operation 100 creates, within the volume of theingot 1, and at a depth close to the depth of penetration of the H⁻ions, a fragile layer dividing the ingot 1 into a lower regionconstituting the mass of the ingot 1 and an upper region constituting alayer 2 of material destined to form the required substrate.

[0071] In the example described here the layer 2 is approximately 10 μmthick. That thickness is sufficient to avoid deformation of the layer(for example the formation of blisters) and the implantation conditionsproduce sufficient fragility at the detachment depth for the layer 2 tobe detached from the ingot 1 with less force.

[0072] The separated layer 2 is advantageously held to enable it to beunwound.

[0073]FIG. 2 shows a device 50 in accordance with the invention forfabricating substrates which implements the method illustrated byFIG. 1. It includes means 110 for implanting H⁻ ions, means 310 forholding the layer 2 when it has been separated from the ingot 1, androtation means 410.

[0074] The implantation means 110 comprise an implanter which producesH⁻ ions accelerated to an energy of the order of 1 MeV. This type ofimplanter was initially developed by the Japan Atomic Energy ResearchInstitute (JAERI).

[0075] The rotation means 410 cause the ingot 1 to rotate about the axisX-X.

[0076] The holding means 310 comprise a support 6. The support 6 isadvantageously an adhesive film. The layer 2 is brought directly intocontact with the support 6. The support 6 is pressed against the layer 2by an applicator roller 320. The applicator roller 320 is mounted on ashaft that is mobile so that it can track the movement of the surface ofthe ingot 1 as its diameter is decreased by the removal of material.Accordingly, when the ingot 1 begins to rotate about the axis X-X,before it has been implanted, no transfer of the layer 2 onto thesupport 6 is effected. Then, when the implantation operation 100 isstarted, and the implanted area comes into contact with the support 6,the latter enables the layer 2 to be transferred onto the support 6.After contact with the layer 2, it separates the latter from theremainder of the ingot 1. The transfer of the layer 2 onto the support 6can then continue.

EXAMPLE 2

[0077] A second example of implementation of the method according to theinvention, shown in FIG. 3, includes an implantation operation 100, aheating operation 200 and a detachment operation 300.

[0078] The species implanted are advantageously hydrogen ions. Hydrogenions are implanted with an energy of the order of 700 keV and a dose ofthe order of 10¹⁷/cm².

[0079] With the ingot 1 rotating continuously about its axis X-X, thearea of the surface of the ingot 1 bombarded by the atomic speciesduring the implantation operation 100 moves towards a heating area.

[0080] The heating operation 200 is carried out after the implantationoperation 100 by heating means 210. The heating operation 200 assistsdetaching the layer 2 between the surface and the detachment depth fromthe remainder of the ingot 1. The heating operation 200 enables thedoses of atomic species implanted to be reduced relative to the dosereferred to in example 1.

[0081] The heating means 210 consist of a heating roller 215 in the formof a circular cylinder with its axis parallel to the rotation axis ofthe ingot 1. The heating roller 215 is placed downstream of theimplantation means 110 relative to the rotation direction of the ingot1. The heating roller 215 is in contact with the ingot 1. The heatingmeans 210 advantageously heat the surface of the ingot 1 locally to atemperature of around 500° C./600° C. The temperature is adjusted tosuit the time of application of the heating means 210 and theimplantation conditions, such as the implantation dose and energy. Thedose and energy parameters also determine the temperature reached by thesurface of the ingot 1 during the implantation operation 100. Thisheating of the ingot 1 by implantation is taken into account in thethermal budget which determines the conditions of detachment of thelayer 2 from the remainder of the ingot 1. The time of application ofthe heating means 210 also depends on the application surface area, therotation speed of the ingot 1, etc.

[0082] The layer 2 is then transferred onto a support 6, as described inexample 1.

EXAMPLE 3

[0083] A third example of implementation of the method according to theinvention, shown in FIG. 4, includes an implantation operation 100 and aheating operation 200, like those of example 2, plus a detachmentoperation 300 performed with the aid of holding means 310. The holdingmeans 310 can employ a pressure difference, an electrostatic force, areversible adhesion force (by application of a low-tack adhesive), etc.

[0084] If the holding means 310 are of the suction type, theyadvantageously comprise a bar 315 with its length parallel to the axisX-X of the ingot 1. The bar 315 is hollow. The pressure inside the baris reduced to hold the layer 2 reversibly by suction. Detachment of thelayer 2 from the remainder of the ingot 1 is encouraged by the heatingoperation 200.

[0085] To begin peeling the layer 2 off the ingot 1, the holding means310 are applied to the first area of the ingot 1 that has been subjectedto the implantation operation 100 and the heating operation 200. Becauseof the suction in the bar 315, mechanical stresses are applied to thelayer 2. Those mechanical stresses are reinforced by movement E of thebar 315 away from the ingot 1. A separation front F is then obtained.

[0086] The holding means 310 are moved by the means shown in FIGS. 5 and6, for example.

[0087] In the embodiment shown in FIG. 5, bars 315 are distributed overthe periphery of a drive roller 316. The principle of FIG. 6 is the sameas that of FIG. 5, but the bars 315 are distributed over a conveyor belt317 moving in a straight line between two drive rollers 316.

[0088] In the embodiments shown in FIGS. 5 and 6 the pressure inside thebar 315 is reduced just before it comes into contact with the ingot 1.At the moment of contact the bar 315 adheres to the surface of the ingot1. If the ingot has not been subjected to the implantation operation 100and the heating operation 200, in the area of contact between the ingot1 and the bar 315 the latter move relative to each other and the contactis broken. On the other hand, if the ingot has been subjected to animplantation operation 100 in the area of contact between the ingot 1and the bar 315, the layer 2 is separated from the remainder of theingot 1 and is held by the bar 315. The vacuum in the bar 315 is brokenwhen the area of the layer 2 held by the bar 315 reaches a take-uproller 8.

[0089] This embodiment has the advantage over the previous embodimentthat the layer 2 is subject to lower mechanical stresses because thereis no reversing of the curvature of the layer 1 between the detachmentoperation 300 and the operation of storing it on the take-up roller 8.

[0090] In a variant of these embodiments of the method according to theinvention, the layer 2 is transferred to a support 6. In this case, thelayer 2 is moved away from the ingot 1 by the holding means 310, aspreviously indicated, and then transferred to the support 6, to which itadheres. The layer 2 is then released by the holding means 310 andentrained by the support 6.

[0091] In this variant the layer 2 can be cut into sheets before orafter it is transferred to the support 6.

EXAMPLE 4

[0092] A fourth example of implementation of the method according to theinvention includes an implantation operation 100, an operation 400 oftransfer onto a stiffener support 6, and a heating operation 200 carriedout before or during the transfer operation 400.

[0093] This is shown in FIG. 7.

[0094] In this example, the implantation operation 100 is carried out atan energy of 100 to 200 keV. This energy is insufficient to produceself-supporting layers 2. The support 6 then acts as a stiffener. Thisprevents the layer 2 breaking and/or deforming (by forming blisters, forexample).

[0095] The support 6 is advantageously an adhesive film. The adhesivefilm consists of a polymer resin, for example, or some other substancesuited to this function, which becomes adhesive when it is heated orwhen it is irradiated with UV radiation. The adhesive film is stretchedbetween two rollers 8, 10 between which the ingot 1 is pressed onto thesupport 6. The axes of the rollers 8, 10 and the ingot 1 are parallel.The support 6 is initially wound onto a pay-out roller 10.

[0096] The ingot 1 is rotated about its axis X-X by rotation means 410.

[0097] In this example the heating means 210 take the form of a roller215. The roller presses the support 6 and the layer 2 together and heatsthem at the same time. The support 6 serves as a stiffener whichprevents deformation of the layer 2 (for example by blisters) that couldotherwise occur during the heating operation 200 that is virtuallysimultaneous with the coming into contact of the layer 2 and the support6. The heating operation 210 strengthens the adhesion between thesupport 6 and the layer 2 and contributes to making the ingot 1 fragileat the detachment depth.

[0098] After adhering to the support 6, the layer 2 leaves the ingot 1at the separation front F. Synchronizing the rotation of the ingot 1with the movement of the support 6 propagates the separation of thelayer 2 relative to the ingot 1 at the front F. At the separation frontF, the material is sufficiently fragile for the mechanical stressesapplied to the ingot 1 by the support 6 to complete detachment.

[0099] If the implantation and heating parameters are chosenaccordingly, the detachment of the layer 2 from the ingot 1 has alreadyoccurred by the time of the heating operation 200 and the support 6merely entrains the layer 2 away from the ingot 1.

[0100] The combination of the layer 2/support 6 is then wound onto atake-up roller 8 for storage.

[0101] As an alternative to the above, the support 6 can be preheatedbefore it is brought into contact with the ingot 1 or the ingot 1 can bepreheated before the support 6 is brought into contact with it.

[0102] In another variant of the embodiment of the method according tothe invention described hereinabove a wedge, blade or some other type ofmechanical contact or a jet of fluid, such as a gas, is used to initiatethe separation of the layer 2 from the ingot 1.

[0103] In a further variant of this embodiment, the support 6 takes theform of a plate 20 (see FIG. 9). The plate 20 is rigid. It is made ofglass or quartz, for example. Thus an implantation operation 100 can becarried out after which the layer 2 is transferred to the plate 20; itcan be heated to facilitate detachment and transfer of the layer 2 ontothe plate 20 and adhesion of the layer to the plate. It is also possibleto interrupt the implantation operation 100 to complete the transfer ofa portion of the layer 2 already implanted onto the plate 20 and thenstart again with a new plate 20.

EXAMPLE 5

[0104] A fifth example of implementation of the method according to theinvention is derived from the fourth example, illustrated by FIG. 7. Thefifth example, illustrated by FIG. 8, includes an implantation operation100, a heating operation 200 and an operation 400 of transfer onto astiffener support 6, as in the fourth example, but further includes anoperation that creates a thermal shock. After the implantation operation100, the ingot 1 is at a relatively high temperature. By pressing acooling roller 216 against the ingot 1 in the areas that have beensubjected to the implantation operation, a thermal shock is producedthat facilitates separating the layer 2 and the ingot 1.

EXAMPLE 6

[0105] A sixth example of the implementation of the method according tothe invention includes an implantation operation 100, a heatingoperation 200 and a detachment operation 300, all of which are of thesame kind as those of example 4. However, it further includes anoperation of covering the layer 2 deposited on the support 6 with acovering material 12. The covering material 12 is deposited in the formof a film or in the liquid or gas phase. This example is illustrated byFIG. 10. A system of three layers 6, 2 and 16 is then obtained. Oneexample of use of an embodiment of this kind is described hereinafter inexample 8.

EXAMPLE 7

[0106] A seventh example of implementation of the method according tothe invention includes an implantation operation 100 which isadvantageously effected simultaneously with H⁻ ions and phosphorus ions(FIG. 11a). With the same acceleration energy, the H⁻ ions are implantedmore deeply than the phosphorus ions, because they are not so heavy. TheH⁻ ions therefore determine the depth at which detachment occurs. Thephosphorus ions produce an n-doped doping layer 16. The layer underlyingthe doping layer 16 forms a p-doped layer 17. FIG. 11b shows the profileof the concentration C of the atomic species H⁻, P₂H₆ and PH₃ as afunction of the depth of implantation in the layer 2 and the ingot 1shown in FIG. 11a. Thus doping and implantation for the purpose ofdetachment can be carried out at the same time.

[0107] This example is advantageously completed by a heating operation200, a detachment operation 300 and a transfer operation 400 asdescribed in example 4.

EXAMPLE 8

[0108] An eighth example of implementation of the method according tothe invention is derived from the sixth and seventh examples describedhereinabove. In the eighth example, shown in FIGS. 12a and 12 b, thesupport 6 and the covering material 12 already incorporate patterns.

[0109]FIG. 12a shows a support 6, a layer 2 and a covering material 12.The support 6 includes interconnection patterns (not shown). The siliconlayer 2 is obtained from a silicon ingot 1 by the method according tothe invention. The layer 2 includes an n-doped layer 16 that isadvantageously doped with phosphorus or arsenic and a p-doped layer 17,as indicated hereinabove in example 7. The covering material 12 alsoincludes interconnection patterns. The combination of the three layers6, 2 and 12 is assembled by the method according to the presentinvention, for example the variant of the method illustrated by FIG. 6.This produces a photovoltaic device like that shown in FIG. 12b, inwhich the face including the covering material 12 is exposed to thephotons 18.

[0110] The covering material 12 provides an antireflection coating. Thesurface of the layer 2 is rough because it has not been polished afterthe detachment operation 300 of the method according to the presentinvention. This enables light 18 to penetrate into the layer 2 withmultiple reflections.

EXAMPLE 9

[0111] In a ninth example of implementation of the method according tothe invention, shown in FIGS. 13a and 13 b, the implantation operation100 is carried out over the whole of the surface of the ingot 1. To thisend the ingot 1 is placed in a plasma implantation chamber in which theatomic species are accelerated to the required voltage (FIG. 13a).

[0112] The ingot 1 is then optionally subjected to a heating operation200, depending on the conditions used in the preceding implantationoperation 100.

[0113] The ingot 1 is then withdrawn from the implantation chamber topeel off the layer 2. The layer 2 is advantageously transferred onto asupport 6, as in any of the preceding examples (FIG. 13b).

[0114] In a variant of this example the ingot 1 is subjected to theother operations leading to the formation of the layer 2 in the samechamber as the implantation operation 100.

EXAMPLE 10

[0115] A tenth example of implementation of the method according to theinvention, shown in FIG. 14, includes an implantation operation 100, aheating operation 200, and a detachment operation 300, conforming tothose described in connection with example 2, but the ingot 1 has asquare cross section relative to its longitudinal axis. The heatingoperation 200 and the detachment operation 300 are carried outsimultaneously by means of a heating roller 215. The heating roller 215is mounted on a shaft that is mobile so that it can track the movementof the faces of the ingot 1 as it rotates and so that it can track themovement of the surface of the ingot 1 as its size is reduced byremoving material.

[0116] Many variants of the method according to the invention can beobtained by combining the various embodiments described hereinabove.

[0117] In the embodiments described hereinabove, the implantationoperation 100 is carried out by bombarding the surface of the ingot 1either with a beam of atomic species or by immersion in a plasma. If abeam of atomic species is used, it can have a linear or rectangularshape or any other geometry. The ingot 1 can also be bombarded radiallyby more than one beam, simultaneously at several points on the surface,or even over the whole of its surface.

[0118] In the embodiments described hereinabove, a heating operation 200can be carried out to encourage and/or cause detachment of the layer 2from the ingot 1. That operation can be complemented by the applicationof mechanical stresses to complete said detachment and separate thelayer 2 from the remainder of the ingot 1. However, the detachment ofthe layer 2 from the ingot 1 can be encouraged and/or caused entirely bythe heating operation 200. It can also be encouraged and/or causedentirely by mechanical stresses.

[0119] Similarly, in the embodiments described hereinabove, the atomicspecies implanted to create microcavities is hydrogen. Other atomicspecies can equally well be used. Examples are helium, boron, etc. Boronis advantageously used to dope the layer at the same time as encouragingor causing detachment. Boron can equally advantageously be used toreduce the doses of the atomic species implanted and/or the temperaturesand/or the times of heating of the optional heating operation 200 (seeU.S. Pat. No. 5,877,070, for example).

[0120] For some applications, and in particular if the surface of theingot 1 exposed to implantation must be protected, a buffer layer can bedeposited on the upstream side of the implantation 100, relative to thedirection of rotation of the ingot 1.

[0121] Similarly, it can be beneficial to deposit a stiffener on theingot 1, even before the implantation operation 100.

[0122] As a general rule, depending on the intended applications, it canbe beneficial to deposit a support 6 (stiffener, reflecting layer, etc.)on one or both of its faces before or after the layer 2 is peeled off.

[0123] In a further variant of the method according to the invention,the layer 2 is transferred temporarily to a support 6 serving as astiffener enabling a detachment operation 300 to be carried out, or evenonly a operation constituting a preliminary to the detachment operation,such as a heating operation 200, preventing deformations such as thosecaused by the formation of blisters. The support 6 advantageouslycarries the layer 2 from the ingot 1 from which it has been obtained tostorage means or carries the layer 2 before it is transferred to anothersupport that confers the required mechanical strength on it. Thus oneface of the layer 2 can adhere temporarily to the temporary support 6,which possibly also serves as a stiffener, after which another supportis caused to adhere to the other face, after which the temporary support6 is finally removed.

[0124] A roller in contact with the ingot 1 downstream of theimplantation means 110 can also serve as a temporary stiffener. This isadvantageously combined with a heating operation 200.

1. A method of fabricating substrates, in particular for optics,electronics or opto-electronics, characterized in that it includes: anoperation (100) of implanting atomic species under the surface of amaterial in the form of a cylindrical ingot (1) at an implantation depthdistributed around a particular value by bombarding said atomic speciesonto an area of the cylindrical surface of the ingot (1), and adetachment operation (300) at a detachment depth in the vicinity of theimplantation depth of the layer (2) of material between the surface andthe detachment depth to detach that layer (2) from the remainder of theingot (1).
 2. A method according to claim 1, characterized in that theimplantation (100) of the atomic species is effected by continuouslybombarding the cylindrical surface of the ingot (1) with a localisedbeam that is swept in the longitudinal direction of the ingot while theingot is rotated about an axis parallel to the cylindrical surface.
 3. Amethod according to claim 1, characterized in that the implantation(100) of the atomic species is effected by continuously bombarding thecylindrical surface of the ingot (1) with a beam of elongate crosssection corresponding to a given area while the ingot is rotated aboutan axis parallel to the cylindrical surface.
 4. A method according toclaim 1, characterized in that the implantation (100) of the atomicspecies is effected by bombardment corresponding to a given area ofsuccessive zones adjacent the cylindrical surface of the ingot (1) whilethe ingot (1) rotates about an axis parallel to the cylindrical surfaceof the ingot (1) between each bombardment and the next.
 5. A methodaccording to any preceding claim, characterized in that the whole of thecylindrical surface of the ingot (1) is bombarded.
 6. A method accordingto claim 5, characterized in that the whole of the cylindrical surfaceof the ingot (1) is continuously bombarded by immersing the ingot (1) ina plasma.
 7. A method according to any preceding claim, characterized inthat it further includes a heating operation (200).
 8. A methodaccording to any preceding claim, characterized in that it includes anoperation of transferring the layer (2) of material between thecylindrical surface of the ingot (1) and the detachment depth onto asupport (6).
 9. A method according to claim 8, characterized in that itincludes an operation of pressing the layer (2) of material between thecylindrical surface of the ingot (1) and the detachment depth onto thesupport (6) by means of a cooling roller (216).
 10. A method accordingto either claim 8 or claim 9, characterized in that the support (6) isadhesive.
 11. A method according to any preceding claim, characterizedin that it includes an operation of covering the layer (2) of materialbetween the cylindrical surface of the ingot (1) and the detachmentdepth with a liquid-phase or gas-phase deposit.
 12. A method accordingto any preceding claim, characterized in that said material is silicon.13. A method according to any preceding claim, characterized in that theatomic species comprise hydrogen ions.
 14. A method according to claim13, characterized in that the atomic species comprise doping ions suchas phosphorus or arsenic ions.
 15. A method according to any precedingclaim, characterized in that it includes operations of applying a layer(6, 12) to each face of the layer (2) of material between thecylindrical surface of the ingot (1) and the detachment depth by rollingthose layers between rollers.
 16. A method according to any precedingclaim, characterized in that it includes an operation of transferring atleast one layer (6, 12) comprising circuitry patterns onto the layer (2)of material between the cylindrical surface of the ingot (1) and thedetachment depth.
 17. A method according to any preceding claim,characterized in that the ingot (1) has a circular cross section.
 18. Amethod according to any preceding claim, characterized in that the ingot(1) has a square cross section.
 19. A device for fabricating substrates,in particular for use in optics, electronics or optoelectronics, whichdevice is characterized in that it includes: means for implantation(110) of atomic species under the surface of a material in the form of acylindrical ingot (1) at an implantation depth distributed around aparticular value, detaching means (210, 310) for detaching a layer (2)of material at a detachment depth in the vicinity of the implantationdepth, and rotation means (410) for rotating a cylindrical ingot (1) ofthe material about an axis parallel to the cylindrical surface of theingot (1).
 20. A device according to claim 19, characterized in that itincludes holding means (310) for holding the layer (2) of materialbetween the cylindrical surface of the ingot (1) and the detachmentdepth to gather up said layer (2) after it is detached from the ingot(1).
 21. A device according to claim 20, characterized in that theholding means (310) include a plurality of suction gripping means (315)distributed over drive roller means (316, 317).